What was terrible about Jekyll and what happened at the window?

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Through one of the windows, Utterson spots Dr. Jekyll, whom he has not seen for weeks. Utterson calls to Jekyll and tells him he should get outside more. Jekyll replies that he wishes he could, but doesn't dare. As he finishes his sentence, his smile disappears from his face and a look of utter terror takes over. It appears as though Jekyll suffers some kind of seizure.